The present invention relates to a nickel-chromium stainless steel comprising nickel-chromium SUS 304 stainless steel as the base, having improved corrosion resistances and machinability and usable particularly as a preferred material for food machines.
The chemical composition of the SUS 304 steel as stipulated by JIS is as shown in Table 1.
TABLE 1 ______________________________________ (wt. %) C Si Mn P S Ni Cr ______________________________________ &lt;0.08 &lt;1.0 &lt;2.0 &lt;0.04 &lt;0.03 8.0.about.10.5 18.0.about.20.0 ______________________________________
Although the SUS 304 steel is widely used as a corrosion-resistant material, its machinability is quite poor. When a free cutting property is required, usually a sulfide inclusion such as MnS was formed purposely at the considerable sacrifice of the corrosion resistance in the prior art. However, it was reported that when the corrosion resistance was regarded as particularly important to make the SUS 304 steel resistant to a strong corrosion environment (for example, a chloride environment or acidic drink environment), it was effective to reduce the ratio of manganese to sulfur (Mn/S) which were the main constituents of MnS in the steel and to increase the amount of chromium dissolved in MnS [see "TETSU-TO-HAGANE (The Journal of the Iron & Steel Inst. of Japan)", 70 (1984), P. 741].
A solid solution treatment at high temperature has been also known to improve the corrosion resistance. For example, when the SUS 304 steel as shown in Table 1 is maintained at about 1300.degree. to 1400.degree. C. for about 1 to 60 minutes and then quenched, the resulting steel exhibits the improved corrosion resistance as compared with the SUS 304 steel treated by a conventional heat treatment at 1050.degree. C. [see Preconference Text of "33rd Conference concerning Corrosion and Anticorrosion" (held on Oct. 15-17, 1986 at Naha, Okinawa, Japan), P. 119-122(1986)].
Though the machinability can be improved to some extent without reducing the corrosion resistance by suitably balancing the formation of MnS with the reduction of the Mn/S or by the solid solution treatment at high temperature, the improvement is yet insufficient.